This invention relates generally to a focussing error detecting apparatus, and more particularly is directed to such apparatus suitable for use in an optical disc reproducing device, an auto-focus camera or image-pickup device, and the like.
In conventional optical disc reproducing devices, a beam of light from a laser light source is converged by an objective lens onto the record surface of an optical disc. The surface of the optical disc has recorded thereon video or audio information in circular or spiral tracks in the form of depressions, e.g. pits, or ridges. The beam is reflected from or transmitted through the disc in such a manner that it is modulated by the pits, and hence in accordance with the information recorded in the tracks. The reflected or transmitted modulated laser beam is then directed onto the detecting face of a photodetector, which generates a reproduced signal in response to the information contained in the modulated light beam. This reproduced signal is further processed and ultimately is reproduced as the video images or audio sounds recorded on the disc.
For accurate reproduction, it is necessary that the laser light beam be focussed exactly on the record surface of the optical disc. When the laser light beam is in this just-focussed state, the pits and ridges in the tracks on the record surface are effective to properly modulate the light beam. However, it may happen that the light beam is under-focussed, that is, the beam is focussed not exactly on the record surface but slightly behind it. In such case, if the objective lens is correspondingly moved slightly away from the disc along the optical axis of the light beam, the light beam would then converge accurately on the record surface. Conversely, the light beam may be over-focussed and converge at a point slightly before the record surface. In this case, the problem may be cured by moving the objective lens slightly closer to the disc, whereby the light beam will then converge exactly on the record surface. These two types of focussing error must be accurately detected and compensated for. To this end, the objective lens is mounted on and controlled by a servomechanism to be movable along the optical axis of the laser light beam towards and away from the optical disc to adjust for the slight under- or over- focussing of the laser light beam.
Focussing error detecting apparatus have been previously proposed for controlling the objective lens in this fashion. In particular, in an optical disc reproducing device using a reflected light beam, the photodetector may be divided into sections or separate photodetecting elements which receive light from separate portions of the reflected light beam. Each section produces a signal in response to the amount or intensity of the light which it receives. These signals are then combined to create a focussing control signal dependent on the state of the light beam, i.e. under-focussed or over-focussed and to what extent. This focussing control signal is transmitted to the servomechanism, which moves the objective lens closer to the optical disc when the light beam is over-focussed, and away from the disc when the light beam is under-focussed.
In this type of focussing error detecting apparatus, various devices have been proposed for directing the reflected light beam onto the separate sections of the photodetector. These devices include, for example, prisms and/or lenses. In these devices, different portions of the reflected light beam pass through different sections of the prism or lens to fall onto the separate sections of the photodetector. Each individual portion of the light beam is therefore affected differently depending on the geometry of the section of the prism or lens through which it passes, e.g. it may be weakened or diffracted. In the various types of devices proposed for this purpose, there has been difficulty in achieving the desired precision and linearity required for highly accurate control of the motion of the objective lens.